Ginny,
There may also be some options to use FileVault encryption on the disk FIRST, wait until it is complete, then booting into Recovery mode, with CMD-R held down, use disk utility to erase the encrypted disk, which is going to delete the encryption keys, so NO ONE would ever be able to access the data.
This is preferable especially on more modern solid state disk (SSD) drives, as the older multiple pass 'secure erase' methods were designed for traditional magnetic rotating disk drive media, but frequently is NOT even an available option for the SSD disks.
The encrypt first, then erase will work on both kinds of media to be VERY secure.
There are several other methods that may be faster or easier too:
or you can do it this way too:
etc...
(I wasn't sure what version of the MacOS you may be using, so, click on the pull down menu with your actual macOS version to get the correct instructions)
Thanks,
Daniel Feldman
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